For example, in recent years, to respond to the demands for improvement of the fuel efficiency of automobiles and enhancement of safety or the like, high strength thin-gauge steel sheets have been used in large amounts in automobile body. Use of laser welding to weld these steel sheets at a faster speed has been sought. Furthermore, when overlaying and laser welding high strength thin-gauge steel sheets, it is preferable that a high strength of the joints can be stably obtained.
Laser welding uses laser beam as a heat source, so enables more reliable and easier control of the heat input compared with arc welding such as TIG welding and MIG welding. For this reason, by suitably setting the welding speed, the irradiation power of the laser beam, the flow rate of the shielding gas, and other welding conditions, the welding deformation can be reduced. Further, laser welding enables welding from one side, so is preferable for welding for assembling complicated members such as the body of automobiles
In recent years, high efficiency remote laser welding using a mirror for positioning a laser beam at a high speed and moving it between weld locations in a short time has been spreading.
In remote laser welding, for example, as shown in FIG. 5, a plurality of weld points of the flange part 50a of a hat-shaped member 50 are laser welded with weld beads 51.
In laser welding, if a load is applied to the weld zone, stress concentrates near the portion where the fusion line of the weld bead (weld interface) and the overlaid part of the steel sheets intersect. For this reason, depending on the magnitude of the load, sometimes this leads to fracture.
PLT 1 discloses a method of dealing with this problem by, as shown in FIG. 4, further forming a second weld bead 61 at the inside of a first weld bead 60 to make the weld bead multiple beads and thereby further improve the joint strength of the steel sheets 5, 5 and tempering the first bead 60 by the heat of the second bead 61. Due to this, the ductility of the weld interface of the weld bead is improved, the concentration of stress at the overlaid part at the time of application of load is eased, and the tolerance to the applied load becomes higher.
PLT 2 discloses the method of forming three beads in the order of the main bead, compression field providing bead, and tempered bead and preventing cracking of the beads.